The present invention relates to a sanding method and apparatus, and more particularly to a method of and an apparatus for efficiently and highly accurately sanding an unpainted or painted surface of a wood component of wood furniture and fittings.
Many pieces of wood furniture and fittings, such as doors and decorative panels, generally include frames having carved or uneven surfaces with elongated straight convexes and concaves. After the surface of a wood frame has been carved by a machine, the carved surface is sanded and painted, and then the painted film is sanded to finish the painted surface smoothly.
FIGS. 1 and 2 of the accompanying drawings shows a conventional apparatus for sanding a carved wood surface and a painted wood surface. As shown in FIG. 1, a workpiece (frame) 4 is placed on a horizontal table 2, and feed rollers 6a, 6b are rotated counterclockwise (FIG. 1) in the direction of the arrows to move the workpiece 4 to the left in the direction of the arrow.
An endless sanding belt 10 trained around rollers 8a, 8b is disposed above the table 2 and extends in the direction in which the workpiece 4 is fed. The rollers 8a, 8b are rotated counterclockwise (FIG. 1) to move the sanding belt 10 in the direction opposite to the direction of travel of the workpiece 4, and at the same time the portion of the sanding belt 10 between the rollers 8a, 8b is pressed down against the workpiece 4 by a pad 12 to sand the workpiece 4. The sanded surface of the workpiece 4 is carved or has an elongate straight convex 4a and an adjacent elongate straight concave 4b, each having an arcuate cross-sectional shape. The pad 12 has a pressing surface 12a (FIG. 2) which also has a concave and a convex that are complementary to the convex 4a and the concave 4b, respectively, of the workpiece 4.
With the conventional sanding apparatus, the width of the sanding belt 10 has to be be larger than the width of the pad 12 so that the sanding belt 10 will not disengage from the pad 12 while the workpiece 4 is being sanded. Therefore, the sand belt 10 has opposite marginal edge portions 10a, 10b of a relatively large width which are not used in sanding the workpiece 4, and hence is quite uneconomical.
Since the pad 12 is pressed against the inner surface of the sanding belt 10 which is being moved by the rotating rollers 8a, 8b, the inner surface of the sanding belt 10 and the pressing surface 12a of the pad 12 are quickly worn to a large extent. To prevent the pad 12 from being worn out soon, it is necessary to apply an abrasion-resistant resin or an abrasion-resistant material such as graphite or the like to the pressing surface 12a of the pad 12. Accordingly, the cost of the pad 12 is high.
Another problem is that because the moving sanding belt 10 is pressed against the workpiece 4 by the pad 12, the workpiece 4 is subjected to large resistive forces applied in the direction opposite to the direction of travel of the workpiece 4. Therefore, a highly powerful actuator mechanism is needed to feed the workpiece 4 in the direction of the arrow on the table 2. Because of the powerful actuator mechanism thus required, the apparatus is also uneconomical and large in size.
Also, in view of the large load forces required to move the workpiece through the sanding apparatus, a large number of rollers, including upper rollers 6a and lower rollers 6b are required. The large number of rollers required, and particularly the upper rollers 6a, make it difficult to provide a sanding belt that can highly, accurately access inner areas of the workpiece. Thus, for example as shown in FIG. 2, an inside surface contour 4c of the workpiece cannot be accurately and evenly sanded.
During sanding operation, the sanding belt 10 is greatly tensioned since the pad 12 is pressed against the inner surface of the moving sanding belt 10 to sand the workpiece 4. While the convex surface 4a of the workpiece 4 can be sanded well, the steep surfaces of the concave surface 4b may not sufficiently be sanded. As a result, the curved surface of the workpiece 4 cannot uniformly be sanded in its entirety. If the concave surface 4b is to be sanded to the extent which is necessary, then the convex surface 4a will be sanded excessively. When the painted surface is sanded, the paint film may locally be removed. Also, it may not even be possible to access and accurately sand inside surface contours 4c in such a curved workpiece.
The pressing surface 12a of the pad 12 is shaped complementarily to the curved surface of the workpiece 4 as described above. However, the configuration of such curved surface tends to vary from workpiece to workpiece, and it would be impossible to keep the sanding belt 10 in neat and reliable contact with the carved surfaces of different workpieces 4 by means of the pressing surface 12a of the pad 12. Use of the pad 12 for sanding the carved surfaces of different workpieces 4 would therefore result in sanding failures.
When the workpiece 4 is sanded by the sanding belt 10, the abrasive of the sanding belt 10 may be loaded and become useless within a relatively short period of time dependent on the sanding speed and the pressure with which the sand belt 10 is pressed against the workpiece 4. When the sanding belt 10 is applied to sand the workpiece 4, the sanding belt 10 is heated to accelerate the deterioration and loading thereof. The heating of the sanding belt 10 is also responsible for thermally modifying the painted surface of the workpiece 4 which is being sanded.
Another known sanding apparatus includes a sandpaper applied to the entire pressing surface of a pad, which is reciprocally moved relatively to a workpiece while the sandpaper is pressed against the surface of the workpiece being sanded.
Since the sanding apparatus causes violent vibration when the pad is reciprocally moved, the sanding apparatus itself needs to have a special vibroisolating structure. The vibration of the sanding apparatus and the degree of resistance to the movement of the pad widely differ between the direction of travel of the workpiece and the opposite direction, with the consequence that the surface of the workpiece cannot be sanded uniformly and smoothly. Since the sandpaper is fixed to the pad itself, the pad must be replaced relatively frequently, and the replacement procedure is tedious and time-consuming. The cost of the sanding apparatus is high because it requires a plurality of replacement pads.